let arr = [2,2,0,0]
let arr2 = [0,2,2,2]
let arr3 = [0,2,4,2]
let arr4 = [0,2,0,0]

// 玩2048游戏，点击左箭头后，会变成，0代表空，用代码实现左箭头功能
// [4,0,0,0]
// [4,2,0,0]
// [2,4,2,0]
// [2,0,0,0]